Show the model answerAttempt it first — that is what makes it stick
5 marks
Describe how a capnometer measures carbon dioxide concentration
What earns the marks5 marks
| Read the wording | How it MEASURES — the infrared analyser, and nothing else |
|---|---|
| The physical principle | CO₂ absorbs infrared at 4.26 µm |
| The components | In the order light passes through them |
| Sources of error | And how each is corrected |
1 · Why carbon dioxide absorbs infrared
Carbon dioxide is a non-elementary molecule whose vibrations change its dipole moment, so it absorbs infrared radiation. Its absorption peaks at 4.26 µm, and clinical analysers use a narrow band of 4.2 to 4.3 µm. Elementary, symmetrical gases — oxygen, nitrogen, the noble gases — have no dipole moment and cannot be measured this way at all.
2 · The law that turns absorption into a number
Where I is the transmitted intensity, I₀ the incident intensity, ε the extinction coefficient of carbon dioxide at that wavelength, c the concentration and l the path length through the sample chamber. Beer's law gives proportionality to concentration, Lambert's law proportionality to path length. The energy absorbed is therefore proportional to the number of carbon dioxide molecules in the beam, and the concentration is derived by comparing the absorbance against a known standard.
3 · The optical bench, component by component
Name them in the order the light travels and the description writes itself. This is the part most candidates could not produce.
| Component | What it does | Why it is built that way |
|---|---|---|
| Infrared source | A heated element — commonly a wire or ceramic filament at 1500–4000 K — radiating a broad infrared spectrum | Broadband because a single source then serves every gas the analyser measures; the wavelength is selected afterwards rather than at the source |
| Chopper or modulator | A rotating slotted wheel that alternates the beam between the sample and reference paths, typically at 20–100 Hz | Alternating lets one detector see both paths, so the output is a ratio; it also converts a steady signal into an alternating one, which is far less prone to electronic drift |
| Optical filter | Selects the narrow 4.2–4.3 µm band, either as a fixed filter or on a rotating filter wheel | Its selectivity determines how much nitrous oxide interference gets through — a broad filter admits the neighbouring N₂O band and the analyser over-reads |
| Sample chamber | A cell of fixed path length l through which the gas flows, with a window at each end | Path length is fixed because it is the l term in Beer–Lambert — if it varied, the same concentration would give a different reading |
| Sapphire windows | Transparent ends of the sample and reference chambers | Ordinary glass absorbs infrared and would itself behave as an unknown absorber. Sapphire is transparent at 4.26 µm and hard enough not to scratch in clinical use |
| Reference cell or path | An identical chamber of the same path length containing a non-absorbing gas | Makes the output a ratio of transmitted intensities. Ageing of the source, drift in the detector and soiling that affects both paths equally then cancel instead of appearing as a change in carbon dioxide |
| Photodetector | Converts transmitted infrared into an electrical signal — a photoconductive or thermopile detector | Sees both paths in alternation because of the chopper, so a single detector suffices and the two paths cannot drift relative to each other |
| Processor and display | Applies Beer–Lambert, corrects for interference, and displays a partial pressure or percentage | Also applies collision-broadening correction once the gas mixture is entered or measured |
Commonly lost: Only about 35% passed this half, against 56% for the question overall — the improvement available is here, not in part (b).
4 · What makes the number trustworthy
The analyser is zeroed against carbon dioxide-free gas, usually room air, which fixes I₀ and compensates for soiled windows and an ageing source; it is calibrated against a mixture of known concentration, which fixes the scale. Zeroing corrects the offset, calibration the gain.
The most examinable error is collision or pressure broadening: nitrous oxide and oxygen colliding with carbon dioxide broaden its absorption peak, so more infrared is absorbed than the carbon dioxide alone accounts for and the analyser over-reads. Nitrous oxide also absorbs close to the carbon dioxide band, adding spectral overlap. Modern analysers correct for both once the gas mixture is entered or measured.
5 marks
Outline the clinical information obtainable from a capnogram
What earns the marks5 marks
| Group the information | Airway, ventilation, circulation, metabolism, equipment |
|---|---|
| This half carried most candidates | The first half is where the marks were lost |
Start with the numbers
| Quantity | kPa | mmHg |
|---|---|---|
| PaCO₂ | 4.7–6.0 | 35–45 |
| PETCO₂ | 4.0–5.3 | 30–40 |
| Pa–ETCO₂ gradient | 0.3–0.7 | 2–5 |
Commonly lost: Only a few gave the normal end-tidal values, and fewer gave the clinical correlation between those and the arterial carbon dioxide tension.
Then give the correlation explicitly: in a healthy anaesthetised patient, end-tidal carbon dioxide is a useful surrogate for arterial carbon dioxide because the gradient is small and stable. The gradient exists because alveolar dead space dilutes the alveolar sample with carbon dioxide-free gas, and it widens with hypovolaemia, reduced cardiac output, pulmonary embolism, high airway pressures, chronic lung disease, increasing age and the upright posture. When it widens, the surrogate stops working.
Then the information itself
| Category | What the capnogram gives you |
|---|---|
| Airway | Confirmation of tracheal intubation by a sustained waveform over six breaths, and continuous confirmation thereafter. Detection of oesophageal intubation, extubation, obstruction and disconnection. |
| Adequacy of ventilation | Continuous trend between blood gases. Hypoventilation raises the plateau with a normal shape and hyperventilation lowers it; both are visible breath by breath. |
| Adequacy of circulation | A fall with unchanged ventilation reflects reduced pulmonary blood flow: embolism, sudden fall in cardiac output, haemorrhage or cardiac arrest. During resuscitation it indicates compression quality and signals return of spontaneous circulation. |
| Metabolic state | A progressive rise resistant to increased ventilation indicates increased production — malignant hyperthermia, thyroid storm, sepsis, tourniquet release. |
| Breathing system integrity | A baseline that does not reach zero identifies rebreathing: exhausted absorber, incompetent valve or inadequate fresh gas flow. |
| Lung mechanics | A slurred phase II and an upsloping phase III with a widened α angle indicate obstruction to expiratory flow, and the trace tracks the response to a bronchodilator. |
| Depth of block and patient effort | A curare cleft in the plateau indicates spontaneous respiratory effort during controlled ventilation. |
| Dead space | With a volume capnogram, estimation of anatomical dead space by Fowler's method and physiological dead space through the Bohr equation. |
Commonly lost: Many did not mention capnography for monitoring the adequacy of ventilation, and the effect of hypo- and hyperventilation. Three sentences and a short table cover it — write them before the more interesting emergencies.
Taking it further
Viva prompts
Why are the windows of the sample chamber made of sapphire?
Answer
Because glass absorbs infrared. A glass window would attenuate the beam before it reached the sample and would itself behave as an unknown absorber. Sapphire is transparent at the working wavelength and hard enough to resist scratching in clinical use.
What is the difference between zeroing and calibrating this device?
Answer
Zeroing exposes the analyser to carbon dioxide-free gas and sets the reference intensity I₀, correcting the offset — it compensates for soiling of the windows and ageing of the source. Calibration exposes it to a mixture of known concentration and sets the gain. An analyser can be correctly zeroed and still read the wrong value, and vice versa.
What determines the response time of a sidestream analyser, and why does it matter in a neonate?
Answer
Transit time along the sampling line plus the rise time of the analyser, conventionally 10% to 90% of a step change. In a neonate the respiratory rate is high and the expiratory time short, so if the total delay approaches the duration of expiration the plateau is never reached and end-tidal carbon dioxide is under-read. Shorten and widen the sampling line, reduce the sample flow so fresh gas is not entrained, or use mainstream sampling.
Your patient's end-tidal carbon dioxide is 26 mmHg. Can you assume the arterial value is about 30?
Answer
Only if the arterial to end-tidal gradient is normal. The assumption fails precisely when it matters — in hypovolaemia, low cardiac output, pulmonary embolism, chronic lung disease or with high airway pressures, all of which increase alveolar dead space and widen the gradient. In those patients the arterial value may be considerably higher, and only a blood gas will tell you.
How does a volume capnogram differ from the trace you normally see?
Answer
The usual trace is a time capnogram — carbon dioxide against time. A volume capnogram plots carbon dioxide against expired volume, which allows the area under the curve to be used quantitatively: anatomical dead space by Fowler's method, and physiological dead space through the Bohr equation using mixed expired carbon dioxide.